(1) Field of the Invention
The present invention relates to power supplies. Specifically the present invention relates to a method and apparatus for sensing current in power supplies.
(2) Prior Art
FIG. 1 illustrates a prior art approach to sensing current in a power supply. This prior art system includes a power supply secondary output block 3 that typically includes a secondary power supply output circuit. A current sense transformer circuit 5 is coupled to the power supply secondary output circuit. Typically, a secondary transformer output filter 9 is coupled to the current sense transformer circuit 5. The output of the secondary transformer output filter 9 is then coupled to a power bus (e.g., 5V power bus). The output of the current sense transformer is a current determined by the transformer's turn ratio.
A rectification and filter circuit 11 is coupled to the current sense transformer circuit 5 for detecting the output of the current sense transformer circuit 5 and converting the current pulses output into a rectified analog waveform. The rectification and filter circuit 11 may be implemented using a diode 31 that is coupled in series with a parallel combination of a resistor 35, and a capacitor 37. Resistor 35 determines the voltage sensed at the current peaks since voltage equals the product of the current and the resistance. The storage capacitor 15 stores energy during the current off time with a time constant of the resistance of resistor 35 times the capacitance of capacitor 37. Because the output of the transformer is a current pulse with a duty cycle set by the power supply of typically thirty percent (30%), the voltage across resistor 35 and capacitor 37 does not reach a peak voltage value of the current times resistor 35. Also, during the off time, resistor 35 will discharge capacitor 37, which results in a ripple voltage. The response to a current step increase is determined by capacitor 37 and the response to a current step decrease is determined by the time constant formed by resistor 35 and capacitor 37. Therefore, this type of detector has limitations in transient response, ripple voltage, and accuracy is affected by the duty cycle of the current pulses, since resistor 35 and capacitor 37 function as an averaging circuit. The duty cycle is affected by the load, as well as, the other outputs. The output of the storage capacitor 37 is coupled to a sense node (I.sub.sense) 16. Typically, a current share controller 17 may be coupled to the sense node 16.
For a description of a current share controller 17 that may be employed in conjunction with the present invention, please see U.S. Pat. No. 5,428,524, entitled "Method and Apparatus for Current Sharing Upon Multiple Power Supplies" by Harold Massie assigned to the common assignee.
A sense bus isolation circuit 19 is coupled to the sense node 16. The output of the isolation circuit is typically coupled to the I.sub.bus. The I.sub.bus is a signal bus having a voltage which represents the output current of the power supply. In a redundant power supply system (i.e., a system having multiple power supplies supplying the same power supply voltage), the I.sub.bus allows the power supplies to adjust their output current to match the largest output current which is represented by the signal on the I.sub.bus.
An isolation circuit 19 typically includes an operational amplifier 45 having an output port that is coupled to a diode 47. The negative terminal of the diode is coupled to the negative terminal of the operational amplifier 45 to provide negative feedback. The circuit couples a positive I.sub.sense signal to the I.sub.bus without affecting the sense node 16. As long as the sense node 16 is more positive than the I.sub.bus, the rectification circuit will apply the sense node voltage to the I.sub.bus. The I.sub.bus is the current share bus and has the I.sub.sense voltage corresponding to the power supply with the highest sense node voltage.
This prior art technique has several disadvantages. First, the peak detection and sample and hold circuit that employs a resistor and capacitor (RC), introduces an RC delay into the current sense operation. This RC delay affects the time before an current sense reading is accurately detectable. Second, the prior art circuit for detecting current has no element for preventing and/or minimizing ripples on the sense waveform due to the discharge of the capacitor 37 by the resistor 35.
Accordingly, a method and apparatus for sensing current is desirable that minimizes RC delay, maintains accuracy over changing duty cycle, and minimizes ripples in the current sense waveform.